1. Field of Invention
The present invention relates to light projectors used for various illumination and lighting applications and in particular to projectors that are used to obtain visual effects, light pattern generation and projection in stage illumination and in architectural, display and similar applications.
2. Discussion of Prior Art
Lighting projectors, e.g., those used in stage lighting, are typically equipped with one or more control devices for controlling intensity or focusing or dimensioning the beam, changing its color, or changing the beam's direction. Modern multiple parameter (automated) projectors include controls for all of these parameters and more.
Although such projectors perform effectively in many applications, they suffer from a number of limitations which, if overcome, could greatly expand the visual effects achievable by the lighting instruments and extend their utility to other environments. To achieve such advances, improvements are required in the beam forming mechanism, in the projection of patterns, in the management of heat associated with the light source, in the control of beam color, and in the noise levels which derive from present cooling techniques. To be effective, these improvements must function in a densely packed, compact and sometimes highly mobile structure housing both very fragile optical and electronic components together with a light source capable of producing oven-like temperatures. (An exemplary application involving a nominal image size of 10 ft. by 10 ft. (100 square feet) calls for brightness in the neighborhood of 100 foot candles thus requiring the projector to produce about 10,000 Lumens.) Moreover, certain types of lighting instruments go on "tour" and must withstand truck transport abuses and the vagaries of the weather.
A number of lighting control applications call for controllable beam shapes and patterns. Performance lighting in stage productions, for example, often requires a variety of different beam patterns and/or shapes. For this purpose, a projection gate is often used to form the desired image across the beam of light. Typically, the projection gates are embodied as shutters or etched masks that function like stencils in the beam path to project a particular beam configuration. Known arrangements, "gobos" for example, often include rotary assemblies that incorporate several pattern generating elements encircling the axis of rotation, along with a drive mechanism for rotating a selected pattern into the beam path.
In such arrangements only a limited number of patterns are available, there is no grey scale, and resolution is also limited. Another inherent limitation in this type of system, associated with its dependence on physical motion, is the rapidity with which a desired pattern can be selected and implemented.
Arrays of liquid crystal pixels are potentially useful as projection gates because of their electro-optic effect, and because a virtually unlimited number of high resolution images may theoretically be synthesized quickly and easily.
Such liquid crystal arrays can be used to create images by selectively placing each individual pixel of the array in a relaxed (light blocking) state, or in an aligned (light transmitting) state, or in a state intermediate between the two extreme positions according to a "grey scale". Selection of a grey level may be obtained by controlling the voltage or other control stimuli that is applied to the pixel, thus controlling the alignment or transmissivity of the associated liquid crystals. Over certain ranges there is a predictable relationship between the applied control stimulus and the extent of alignment among the liquid crystals in the pixels, thus providing grey scale control. Whether used in this manner or in a two-state, on-off mode, pixellated liquid crystal arrays have the potential to be used in a "light valve" capacity to create a complete picture across a beam of light.
Pixels in an array of liquid crystals may be relatively densely packed thus offering opportunities for higher resolution and transmission efficiency. Also, they may be individually controlled by a addressing scheme capable of selectively placing each pixel of the array in a desired state. Thus a virtually limitless range of images may be rapidly varied. In many applications pixels are arranged in a row and column configuration and activated by applying a potential to a particular control element associated with each of the pixels. Alternatively, a multiplex or other addressing scheme can be employed to reduce the number of elements necessary to address the pixels. Both active and passive matrices may be utilized.
Certain types of liquid crystal arrays have been previously used with some success in image projection applications. Arrays of twisted nematic liquid crystal (TNLC) have been used and have provided several advantages over other image forming techniques. However, TNLC devices typically require pre-polarization of incident light. Since a polarizer has to be placed in the optical path to polarize the light before it reaches the TNLC gate, there is a loss of intensity of more than fifty percent before it even reaches the array. In high intensity projectors for stage lighting and the like, this loss is far beyond acceptable levels.
There have been efforts to address the light loss problem. An improved method of illuminating a TNLC light valve with linearly polarized light is discussed in "Large-screen Projection Displays II" by William P. Bleha, Jr. (S.P.I.E. Vol. 1255, 1990). The disclosed method for converting unpolarized light into linearly polarized light is said to double the intensity realized by conventional polarizers.
The disclosed polarization method uses a polarization convertor consisting of a polarizing beam splitter, a polarization direction rotator and a synthesizer to significantly improve the illumination efficiency. The polarizing beam splitter separates the incident light into two mutually perpendicular linearly polarized beams (transmitted p-polarized light and reflected s-polarized light). The polarization direction rotator effectively recaptures much of the light that was lost in previous polarizing systems by rotating the polarization direction of the p-polarized light ninety degrees to equalize both polarization directions. Thereafter, the two components of the light are combined on the liquid crystal by the synthesizer.
The polarization convertor may ultimately provide a conversion efficiency approaching 100%. However, reflection and absorption losses in the polarization convertor components, plus the losses in the contrast-enhancing sheet polarizer, presently result in an overall 20% loss of intensity as the unpolarized light is converted to a linearly polarized beam.
There are other formidable barriers in addition to excessive light loss. Conventional polarizers typically associated with liquid crystal arrays lose light intensity through an absorption process. Unfortunately, absorption converts light energy into heat causing the temperature of the gate and surrounding optics to rise to intolerable levels. In performance and display applications, where projector temperatures can reach combustible levels, this process of heat absorption causes a thermal buildup which would greatly exceed the temperature limits of the liquid crystal array.
Various cooling techniques have been proposed which have attempted to alleviate the destructive thermal effects of radiant energy absorption. U.S. Pat. No. 4,739,396 to Gilbert Hyatt, particularly columns 50 through 62 of this patent, discusses numerous cooling techniques which have been proposed for use in light projectors. See also U.S. Pat. No. 4,763,993 issued to James H. Vogeley, et al.
Cooling by forced air is thought to be effective in some applications because it is theoretically transparent to incident light and does not reduce the amount of transmission. Unfortunately however, heat dissipation techniques which depend on fan operation and other forced air cooling techniques can create noise levels which make this technique unacceptable for many performance and display applications. Air cooling also exposes optical elements to atmospheric contaminants which can compromise optical clarity and cause other problems.
Other heat control arrangements for use in lamp environments are known which might serve to protect a liquid crystal gate. For example, improvements in the maintenance of a stable thermal environment in stage projectors have also been proposed by providing a heat exchanger that circulates a cooled fluid through a component of the lamp system for additional cooling. The cooled fluid acts in a heat sink capacity to absorb heat and conduct it away from the heat intolerant devices.
These and related techniques for improving the thermal environment of an optical projector system have been described by George Izenour in U.S. Pat. Nos. 4,843,529 and 4,890,208. In those references, a multi-layer dielectric interference filter, otherwise known as a dichroic "hot mirror", and a liquid cell are placed in the light path between the light source and the mechanical projection gate to remove energy in the infrared region from the beam of light. The hot mirror aids the process of infrared filtering by reflecting "near" infrared energy having wavelengths in the range of about 700 to 1500 nanometers while passing light in other regions. The water in the liquid cell is effective in absorbing the "far" infrared energy in the 1.5 to 10 micrometer region that is not reflected by the dichroic filter. The water cell is effective because it displays good transmission characteristics for energy having shorter wavelengths (i.e., visible light).
The combination of the infrared-absorbing liquid cell and the infrared-reflecting "hot mirror" removes infrared radiation from the beam before it reaches the projection gobo. This process reduces the heating effects of the infrared energy and results in an overall increase in the temperature stability of the optical apparatus.
These and other methods of cooling which can include combinations of radiation, convection, and conduction have been employed in reducing the heating effects in some lighting applications. However, the practical utility of these techniques to protect heat absorbing, temperature sensitive liquid crystal light valves in the oven-like environment of a high-intensity projector, has not been demonstrated.
Because of these environmental obstacles and the loss of substantial light through the use of polarizers, the full potential of liquid crystal optics in the projection fields of interest has not heretofore been realized.
A second class of liquid crystal devices is available which will scatter, rather than absorb incident light while in a blocking mode. These scattering liquid crystal arrays thus offer the potential for use in high-intensity projectors having high heat environments.
Descriptions of liquid crystal devices that will scatter incident light are found in U.S. Pat. No. 4,671,618 to Wu et al. and U.S. Pat. No. 4,688,900 to Doane et al. These patents disclose the us of Polymer Dispersed Liquid Crystal (PDLC) arrays which are essentially microdroplets of nematic liquid crystals dispersed in a polymer matrix. The PDLC material is formed by a process known as phase separation. This type of device will scatter incident light when the light encounters a mismatch between the indices of refraction at the media boundaries that are encountered by the light traveling through the matrix. This occurs when the liquid crystals are in a non-aligned state; the mismatch at the boundary of the polymer matrix and liquid crystal material causes the incident light to be reflected, refracted, and thereby scattered.
A similar type of scattering material is described in U.S. Pat. No. 4,435,047, to James L. Fergason, which discloses a liquid crystal material encapsulated in a containment medium. This device allows for transmission of incident light when the indices of refraction are matched and the liquid crystals in the capsules are in an ordered alignment. Scattering or absorption of incident light results from the mismatch of the indices of refraction and the distorted or curvilinear alignment of the liquid crystals in the containment medium.
When light-scattering liquid crystals are placed in an ordered state, e.g., by application of an electric field, the incident light traveling in a direction parallel to the electric field will encounter a virtual match between the indices of refraction at the interface of the liquid crystals and the matrix. The matched indices allow for unimpeded transmission of the incident light in this ordered state. (Thermal, magnetic, optic and other energy sources may also serve to control the alignment of the liquid crystals in an array.) Over a range, the degree of transmission is proportional to the intensity of the applied field, thus affording a grey scale mode of operation.
In addition to offering the ability to scatter rather than absorb incident light, light scattering gates offer the vitally important property of not requiring pre-polarization of the light incident upon the projection gate. This eliminates a light intensity loss which is prohibitive for many applications.
Although there is reduced absorption of light in the visible region, the absorption characteristics for these light scattering gates are not as favorable at longer wavelengths. They are thus subject to thermal damage from absorption of infrared energy in the high-intensity environment of certain projectors. Accordingly, unless provided with an effective cooling technique, the resolution, speed and image forming abilities of scattering liquid crystal arrays can not be exploited in the environment of high-intensity production lighting devices.
It is accordingly an object of the invention to provide a thermally protected lighting projector gate having the resolution, programmability and response time of a liquid crystal array.
It is a further object of the invention to provide a light projection system which exploits the considerable advantages of light-scattering liquid crystal arrays while protecting such arrays from heat damage.
It is a further object of the invention to provide precise beam pattern control for use in lighting and animation applications by utilizing a dynamically variable projection gate with a diverse assortment of patterns for high speed image projection.
Yet another object of the invention is to provide improved projector cooling techniques that provide more stable thermal environments thereby permitting a broader range of beam control devices to be used in high intensity light projectors. Improved cooling to minimize the potential discomfort that is ordinarily experienced by a performer under bright lights is another object.
A further object of the invention is to create innovative and unusual visual lighting effects by jointly and individually utilizing a dynamically variable liquid crystal projection gate and a gobo unit.
An additional aim of the invention is to provide a lighting projector that operates in a closed environment, thereby improving reliability, decreasing the risk of contamination and reducing the noise emanating from the system.
Another goal of the invention is to provide color control assemblies capable of controlling color parameters in a graduated manner in order to accommodate the special characteristics of human color perception.
A further object of the invention is to provide improved color control systems employing feedback supplied by a beam spectrum sensor.
It is another object of the invention to provide improved gobo utilization including a wider range of effects incorporating both static and dynamic patterns.
Another object of the invention is to provide improved dimming means coupled with an intensity level feedback sensor.
Other objects and advantages of the invention will become apparent in the description which follows.